Retroreflective lens

ABSTRACT

A retroreflective lens includes: a single positive power lens element with an index of refraction N 1  and thickness T 1  ; a single negative power lens element with an index of refraction N 2  and thickness T 2 , the negative power lens element being spaced from the positive power lens element by a distance S 1  ; and a reflective surface intercepting light propagating from the negative lens element and reflecting this light toward the negative lens element. The lens elements in combination with the reflective surface provide the retroreflective lens with an F-number between F/3 and F/6. According to one embodiment N2&gt;1.6 and N1-N2&gt;0.15. According to another embodiment of the present invention the single negative power lens element has an integral reflecting surface and the retroreflective lens satisfies the following inequalities: N2&gt;1.6, N2-N1&gt;0.12. According to one embodiment of the present invention T1+T2+S1&lt;0.15 F, where F is the focal length of the retroreflective lens.

CROSS REFERENCE TO RELATED APPLICATIONS

Cross reference is made to commonly assigned, copending U.S. applications Ser. No. (Attorney Docket 77004), filed concurrently herewith and entitled "IMAGING APPARATUS FOR A PHOTOGRAPHIC FILM IMAGE SCANNER", Ser. No. (Attorney Docket 77006) filed concurrently herewith and entitled "ILLUMINANT HEAD ASSEMBLY FOR FILM IMAGE SCANNER", Ser. No. (Attorney Docket 77289), filed concurrently herewith and entitled "ULTRAVIOLET CURABLE RIVETING OF PRECISION ALIGNED COMPONENTS", Ser. No. (Attorney Docket 77805) filed concurrently herewith and entitled "A METHOD FOR ASSEMBLING A FILM SCANNER", and Ser. No. (Attorney Docket 77900) filed concurrently herewith and entitled "A COMPACT DOCUMENT SCANNER".

1. Field of the Invention

This invention relates to fast, high resolution retroreflective lenses suitable for use in an imaging system of a high resolution scanner.

2. Background of the Invention

U.S. Pat. No. 4,241,990 discloses a copying device with a retroreflective lens. A retroreflective lens is constructed of at least one refractive lens element and a mirror. The light rays emanating from an object pass through the lens element(s) and strike the mirror. The mirror reflects the light back through the refractive element(s) to an image plane that lies on the same side of the lens as the object. This type of a lens is more compact than single-pass lenses. (By single-pass imaging lens we mean that the light rays can pass through the imaging lens only once.) U.S. Pat. No. 4,241,990 provides no structural parameters (such as radii of curvature or indices of refraction), nor performance parameters for the retroreflective lens. However, because the image is placed on a photosensitive drum and the disclosed copying devices use lasers and a fluorescent lamp (such lamps typically provide an output of about 20-40 watts) as a light source, the retroreflective lens probably has a high F-number (F/12 or higher).

When an illumination source is not bright, providing an output of less than 1 watt, the imaging lens must be much faster (i.e. smaller f-number, less than f/8 for example). Therefore, a retroreflective lens such as that disclosed in U.S. Pat. No. 4,241,990 is unsuitable for use in a system that has an illuminator of low brightness (1 watt or less).

Furthermore, because copying devices of the type disclosed in this patent operate at about 300 DPI (dots per inch), the resolution of the imaging system is about 150 cycles/inch (6-7 cycles/mm) or less. Because of this relatively low resolution requirement, the imaging lens of such copying device has a large depth of focus and the image will stay in focus even if the focal length of a manufactured imaging lens deviates somewhat from its design specified value. However, such an imaging lens is not suitable for use in applications that require high resolution imaging (50-200 or more cycles/mm).

SUMMARY OF INVENTION

According to one aspect of the present invention a retroreflective lens includes:

a single positive power lens element with an index of refraction N₁ and thickness T₁ ; a single negative power lens element with an index of refraction N₂ and thickness T₂, the negative power lens element being spaced from the positive power lens element by a distance S₁ ; and a reflective surface intercepting light propagating from the negative lens element and reflecting this light toward the negative lens element; wherein N2>1.6, N1-N2>0.15, and the lens elements in combination with the reflective surface provide the retroreflective lens with an F-number between F/3 and F/6.

According to another aspect of the present invention a retroreflective lens includes a single positive power lens element with an index of refraction N₁ and thickness T₁, and a single negative power lens element with an integral reflecting surface, an index of refraction N₂ and thickness T₂. The negative power lens element is spaced from the positive power lens element by a distance S₁. The retroreflective lens satisfies the following inequalities N2>1.6, N2-N1>0.12, and the lens elements in combination with the reflective surface provide the retroreflective lens with an infinity F-number between F/3 and F/6.

According to an embodiment of the present invention the retroreflective lens has an aperture stop located in a vicinity of the reflective surface, the positive power lens element is biconvex and the negative power lens element is biconcave.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A schematically illustrates a prior art document scanner.

FIG. 1B schematically illustrates an embodiment of a document scanner.

FIG. 1C schematically illustrates another embodiment of the document scanner.

FIG. 2 is a side view of the first embodiment of the imaging system of the document scanner. This figure also shows a plurality of baffles.

FIG. 3 is perspective view of the imaging system of FIG. 2, but without the baffles.

FIG. 4 is a side view of the second embodiment of the imaging system of the document scanner. This figure also shows a baffle located between the document and a CCD.

FIG. 5 is a perspective view of the imaging system of FIG. 4.

FIG. 6 is a top view of the imaging system of FIG. 4.

FIG. 7 is a side view of the third embodiment of the imaging system of the document scanner. This figure also shows a baffle shielding the CCD.

FIG. 8 is perspective view of the imaging system of FIG. 7, but without the baffle.

FIG. 9A illustrates an illuminated document and an imaging system.

FIG. 9B depicts imaging lens of the imaging systems shown in FIGS. 2-8, with light rays traversing through the imaging lens.

FIG. 9C depicts another example of an imaging lens.

FIGS. 10A-10C are plots of ray intercept curves of the imaging lens of FIG. 9B.

FIG. 10D is a plot of the field curves of the imaging lens of FIG. 9B.

FIG. 10E is a plot of the distortion of the imaging lens of FIG. 9B.

FIG. 10F is a plot of the lateral color of the imaging lens of FIG. 9B.

FIG. 11 is a plot of through focus polychromatic MTF, at 31 cycles per mm, of the imaging lens of FIG. 9B.

FIG. 12A is a plot of the polychromatic MITF of the imaging lens of FIG. 9B at 0%, 70%, and 100% of full field in both sagittal and tangential planes.

FIG. 12B is a plot of the polychromatic NMTF of the imaging lens of FIG. 9C.

FIG. 13A is a graph of a fractional focal length change vs. fractional magnification change in a document scanner imaging system that utilizes a single-pass lens.

FIG. 13B is a graph of a fractional focal length change vs. fractional magnification in a document scanner imaging system that utilizes a retroreflective lens.

FIG. 14 illustrates a cross-section of a housing, including a plurality of positioning features and a plurality of baffles for shielding the CCD from spurious light.

FIG. 15A is an external, perspective view of the housing of FIG. 14 and a base for supporting the housing.

FIG. 15B is an interior view of the housing part 10A shown in FIG. 15A.

FIG. 15C is an interior view of the housing part 10B shown in FIG. 15A.

FIG. 16A is a perspective view of a portion of the housing with the document positioning fixture in a form of two rails.

FIG. 16B is side view of a portion of the housing shown in FIG. 16A.

FIG. 16C illustrates a portion of the housing shown in FIG. 16A and an illuminated document.

FIG. 16D illustrates an alternative embodiment of the document positioning fixture.

FIG. 16E illustrates another alternative embodiment of the document positioning fixture.

FIG. 17 illustrates a lens barrel, a housing section having a bore for receiving the lens barrel and fingers for holding the lens barrel.

FIG. 18A is a flow chart depicting a procedure for the alignment of the imaging system.

FIG. 18B illustrates a target for alignment of the imaging system.

FIG. 19 illustrates a section of the housing attached to the CCD.

DETAILED DESCRIPTION OF EMBODIMENTS

Document Scanner

A document scanner 1 for imaging a document 2 (at least partially located on an object plane 3) onto an image plane 5 is shown in FIGS. 1B and 1C. The term document is defined to include film, paper or another hard medium bearing viewable information.

The document scanner 1 includes a housing 10 and an imaging system 20 contained in the housing 10. The housing 10 is a light tight container for the imaging system 20. The imaging system 20 includes an imaging lens 25, and a photosensitive medium such as CCD 40. Optional felding mirrors 30 may be included between the imaging lens 25 and the CCD 40, or elsewhere in the imaging system. The housing 10 also includes positioning features 51 for providing proper positioning and support for the document 2, positioning features 52 and 53 (not shown) for positioning and supporting the, imaging system components, and baffles 55. The positioning features are preferably formed integral to the housing 10. They will be discussed in detail later on in the specification.

The document 2, film for example, is moved across the object plane 3 and is imaged (one line at a time) by the imaging lens 25 onto an image plane 5, preferably occupied by a trilinear (color) CCD 40, The length of the CCD 40 is sufficient to capture the width of the image on the document 2, and the document 2 is advanced relative to the document scanner 1, in a direction other than parallel (preferably perpendicular) to the length of the CCD, while the CCD successively captures document image data row after row. In so doing, a two dimensional image of the document is acquired. The document scanner also includes a processor for transferring the image information from the photosensitive medium to a receiver. The processor includes an analog to digital converter 60. The analog signal generated by the CCD 40 is processed and converted (by an analog to digital converter 60) to digital data representing the image. The digital image data can be stored or converted back to an analog image for display as needed.

Three embodiments of the imaging system 20 are shown in FIGS. 2-8. More specifically, FIGS. 2 and 3 correspond to a first embodiment of the imaging system. FIGS. 4-6 correspond to a second embodiment of the imaging system 20. FIGS. 7 and 8 correspond to a third embodiment of the imaging system 20. It is preferred that the imaging systems have a magnification M with an absolute value of about one (=1). The absolute value of the magnification of the first two embodiments is slightly greater than one (|M|=1.1), while in the third embodiment it is slightly smaller than one (|M|=0.9). The imaging systems 20 of the three embodiments depicted in FIGS. 2-8 utilize the same imaging lens 25. The light from the object plane 3 passes through the imaging lens 25 twice before the light strikes the CCD 40. The distances between the object plane 3, the image plane 5, and the imaging lens 25 are fixed. Depending on a specific configuration and constraints on a special envelope within the housing 10, the imaging system 20 may or may not utilize one or more folding mirrors. The first and the second embodiments of the imaging system utilize a folding mirror 30, while the third embodiment does not. Such folding mirrors can be placed between the object plane and the imaging lens and/or the imaging lens and the image plane in order to fold the imaging system 20 so as to make it more compact. (see FIGS. 2-6). The three embodiments depicted in FIGS. 2, 4, and 7 illustrate that different baffle configurations may be used to shield the CCD from the spurious (i.e. unwanted,) light. Spurious light is discussed in the "Baffles" section of the specification.

Imaging Lens

Referring again to FIGS. 1B and 1C, the document 2 is illuminated either in transmission or reflection by an illumination system 70. According to this embodiment the illumination system 70 provides less than 1 watt of power. It is preferred, for reasons of cost and efficiency, that the illumination system provides less than 0.5 watts of power. It is preferred that a narrow line 71 of the document plane is illuminated during the scanning process, such that the long dimension of the line 71 is perpendicular to the length of the document 2. (See FIG. 9A). Light from the illuminated document 2 is propagates toward the imaging lens 25. The imaging lens 25 is a retroreflective lens. One embodiment of this lens includes two lens elements 25A, 25B and a reflective surface 25C. This embodiment is depicted in FIG. 9B. Another embodiment of the retroreflective lens is shown in FIG. 9C. The retroreflective lens 25 intercepts and refracts the light directing it through lens elements 25A, 25B toward the reflective surface 25C. An aperture stop ST of the retroreflective lens 25 is located at or near the reflective surface 25C. Upon reflection from the reflective surface, the light once again traverses the same set of lens elements 25B and 25A in a generally opposite direction from the entering direction. Finally, the light is focused by these lens elements 25A and 25B upon the image plane 5, occupied by a photosensitive medium, preferably a CCD 40. The reflective surface can be a piano mirror as shown in FIG. 9B. Alternatively the reflective surface can be incorporated on the rear surface of the second lens clement, 25B, so that it is curved.

As can be seen from the FIGS. 9B and 9C, lens element 25A is a biconvex lens element. Lens element 25B is a biconcave or meniscus lens element. The focal length of the retroreflective lens 25 is 22.4, and its total length (including the distance from lens element 25B to the reflective surface 25C) is less than 0.2 times the focal length of the retroreflective lens. Its F-number (F/#) of the retroreflective lens 25 is F/4.26 and its magnification (M) is -0.9. (The negative sign indicates an inversion of the image relative to the object.) The specific parameters for the retroreflective lens shown in FIG. 9B are provided in Table 1. The specific parameters for the retroreflective lens shown in FIG. 9C are provided in Table 2. In these tables the radii of curvature, air spaces and thicknesses of the lens elements are provided in millimeters. Nd stands for the index of refraction and V stands for the Abbe V number.

                  TABLE 1                                                          ______________________________________                                         *LENS DATA                                                                                     THICK-                                                         SRF   RADIUS    NESS      MATERIAL   Nd   V                                    ______________________________________                                         OBJ             43.960521 AIR                                                  S1    9.600234  1.561812  LASFN30    1.808                                                                               40.8                                 S2    -28.780036                                                                               0.670771  AIR                                                  S3    -13.466258                                                                               0.520604  SF2        1.648                                                                               33.9                                 S4    13.145601 0.702812  AIR                                                  AS, S5          -0.702812 REFL                                                 S6    13.145601 -0.520604 SF2        1.648                                                                               33.9                                 S7    -13.466258                                                                               -0.670771 AIR                                                  S8    -28.780036                                                                               -1.561812 LASFN3O    1.808                                                                               40.8                                 S9    9.600234  -37.492858                                                                               AIR                                                  S10             -0.700000 S-NSL5 (O'Hara)*                                                                          1.525                                                                               59.8                                 S11             -1.090000 AIR                                                  ______________________________________                                    

It is noted that surfaces S6-S9 are identical to surfaces S1-S4, except that they occur in reverse order. The table also indicates that the aperture stop is located between surfaces S4 and S6, and is located at the reflective surface. Surface S10 corresponds to a cover glass on the CCD array.

                  TABLE 2                                                          ______________________________________                                         *LENS DATA                                                                                     THICK-                                                         SRF   RADIUS    NESS     MATERIAL   Nd    V                                    ______________________________________                                         OBJ             43.9605  AIR                                                   S1    11.06499  3.1230   LLF6       1.53172                                    S2    -50.04128 0.3793   AIR                                                   S3    -12.45649 0.75     SF2        1.64769                                    AS, S4                                                                               -105.641  -0.75    REFL, SF2  1.64769                                    S5    12.45649  -0.3793  AIR                                                   S6    -50.04128 -3.1230  LF6        1.53172                                    S7    -11.06499 -37.44111                                                                               AIR                                                   S8              -0.700000                                                                               S-NSL5 (O'Hara)*                                                                          1.5249                                                                               59.8                                 S9              -1.090000                                                                               AIR                                                   ______________________________________                                    

The imaging lens 25 is located approximately two focal lengths away from the object plane 3, such that the absolute value of the lateral magnification of the imaging system 20 is approximately unity (|M |=1). Acceptable departure from unity magnification is dictated by the image quality required by the imaging application. It is preferable that the absolute value of the magnification M be 0.6<|M|<1.4. It is even more preferable that 0.8<|M|<1.2. It is most preferable that 0.9<|M|<1.1. Maintaining a magnification near unity assures that coma, lateral color arnd distortion aberrations of the imaging lens 25 are reduced or eliminated. The larger is the departure from the unity magnification, the harder it is to control these aberrations. The aberration curves illustrating the performance of the retroreflective lens of FIG. 9B are provided in FIGS. 10A to 10F. More specifically, FIGS. 10A-10C depict rim ray intercept curves for blue, green and red light at several relative field (of view) positions. FIG. 10A corresponds to axis, FIG. 10B corresponds to 0.5 field and FIG. 10C corresponds to full field. The vertical axis depicts aberrations in millimeters. The horizontal axis is relative pupil coordinates the edges being ±full pupil (i.e., ±1.0 pupil). FIG. 10D depicts the field, curvature in the sagittal (S) and tangential (T) fields. FIG. 10E depicts percent distortion at the primary wavelength (546.1 mm across the field). FIG. 10F depicts primary (P) and secondary (S') lateral color (440-650 nm/across the field). FIG. 11 depicts heterochromatic through focus MTF of the imaging lens of FIG. 9B (modulation transfer function) at the frequency of 31 cycles/mm at three field points: axis, 0.7 relative field, and full field. Wavelengths used were 450 nm, 546 nm and 656 nm with respect to chromatic weights of 1, 1, 1. FIG. 12A is a plot of the polychromatic MTF of the imaging lens of FIG. 9B at 0%, 70%, and 100% of full field of view in both sagittal (S) and tangential (T) planes. As FIG. 12A shows, the MTF value of about 0.5 occurs at 70 cycles/mm at the image plane. Thus, the frequencies transferred from the object plane to the image plane can be as high as 120 cycles/mm (0.9×2×70 cycles/mm). Therefore, the retroreflective lens provides resolution sufficient for scanning film and similar documents. FIG. 12B is a plot of the polychromatic MTF of the imaging lens of FIG. 9C at 0%, 70% and 100% of full view of view in both sagittal (S) and tangential (T) planes.

I had modeled an imaging system (of a document scanner) with a fixed position, single-track imaging lens and analyzed the effects of the focal length deviation from its predetermined value on the imaging system magnification M. A fractional focal length change Δf was defined as Δf=(fa/fd)-1, where fa is the actual focal length of the manufactured imaging lens and fd is the predetermined (i.e., design) value of the focal length. A fractional magnification change ΔM was defined as ΔM=1-Ma/Mp=1-(fa/(d-fa))/Mp, where Ma is the actual magnification, Mp is the predetermined (i.e., desired) magnification, and d is an object distance of the imaging lens. The results were plotted in FIG. 13A. As can be seen from this figure, a fractional focal lens change of |Δf|=0.2 results in the fractional magnification change ΔM of 0.04.

I had also modeled an imaging system with a fixed position retroreflective imaging lens of the same focal length (22.4 mm) and again analyzed the effects of the focal length deviation from its predetermined value on the imaging system magnification M. The results are provided in FIG. 13B. I was surprised to discover that that the magnification of this imaging system is very insensitive to the focal length deviations. More specifically, a fractional focal length change of |Δf|=0.2 results in the fractional magnification change ΔM of 0.002. This is a factor of 20 smaller than the corresponding change of single pass system. Thus, I realized that if a retroreflective imaging lens 25 is used in a document scanner, one does not need to do a magnification adjustment of the imaging system 20, because even if the focal length of the retroreflective imaging lens 25 deviates somewhat from its design value, the magnification change of the imaging system is insignificant (i.e., it will be well within ihe acceptable tolerance limits). Therefore, the assembly of the imaging system of the document scanner that utilizes a fixed position retroreflective imaging lens can be done faster and is less expensive then the assembly of an imaging system that utilizes a fixed position single-pass imaging lens. In addition, a retroreflective lens utilizes only half the number of the lens components utilized in a single-pass imaging lens of equivalent optical performance, making the production of the retroreflective lens less expensive. Finally, because the imaging system that utilizes a retroreflective imaging lens fits into a smaller spatial envelope, imaging system 20 can be enclosed in a much smaller housing than the similar imaging system that utilizes a single-pass lens. A smaller housing is more structurally rigid than a similar larger housing. Thus, the use of a retroreflective imaging lens 25 in an imaging system 20 results in a more rigid housing and a more stable optical performance.

In addition, in order to provide a high resolution (50-200 cycles/mm) it is preferable to have a retroreflective lens with the low F-number (F/#) of 3≦F/#≦8. It is even more preferable that 3.5≦F/#≦6. It is most preferable that 4≦F/#≦5. If the lower limit is exceeded the optical aberrations of the retroreflective lens are difficult to correct with only two lens components. Although a retroreflective lens can have more than two lens components, it increases the cost of the document scanner. If the upper limit is exceeded, the resolution of the retroreflective lens begins to deteriorate and the retroreflective lens is less capable of operating with a weak illumination source. In the above embodiment the retroreflective lens has an F-number of F/4.4.

Baffles

If diffuse light is used to illuminate the document 2, the light emanating from the document plane will spread and some of the light will not enter the imaging lens 25. Some of the light which does not enter the imaging lens will reflect off the components of the imaging system and may impinge on the CCD 40, contributing to noise and deteriorating the image quality. This light is called spurious light. This spurious light reduces the contrast of the captured image. It is necessary, therefore, to minimize the amount of spurious light incident upon the CCD. This is accomplished, for instance, with the use of light absorptive 55 (depicted in FIG. 14) within the housing 10 to trap spurious light before it can find its way to the CCD 40. The placement of these baffles 55 is dependent on the specific geometric configuration of the imaging system. It is preferred to place the entire imaging system (from the object plane to the CCD 40) in a light absorptive housing. (See FIGS. 14 and 15A) This ensures that no light from extraneous sources impinges on the CCD 40.

The Housing

The housing 10 is made of two parts 10A and 10B and has a slit 10C for admitting light that illuminated the document 2 into the housing 10. After the imaging lens 25 and the CCD 40 are fixedly placed into one of the housing parts, the other housing part is attached to the first housing part to enclose the imaging system 20. The housing 10 also has one or more apertures 10D, 10E. (See FIGS. 15A-C). The function of these apertures will be discussed later in the specification.

The housing 10 provides positioning features 51, 52 and 53 to precisely position the document 2 relative to the imaging lens 25, and to precisely position the CCD relative to the imaging lens 25. (See FIGS. 15A-15C and 16A-16E). This housing 10, including the positioning features 51, 52 and 53, is preferably fabricated of molded plastic to decrease the cost of the housing and to enable the positioning features 51, 52 and 53 to be molded into the housing 10. Additionally, the housing 10 can readily be molded with flanges 75 that enable it to be mounted to a base 80 that also mounts other subassemblies of the document scanner (for example, electronics, mechanical components for transport of the document). It is therefore desirable to make the housing 10 as rigid as possible to retain the position of the positioning features 51-53 and thus the document, imaging lens, and CCD, despite stress induced by the base 80, and despite the changes due to heat and humidity. Rigidity of the housing 10 is aided by minimizing the physical size of the housing (relative to the thickness of the wall of the housing 10), as well as by making the housing as monolithic as possible. The housing 10 of FIG. 15A has a length of only about 40 mm.

As stated above, the positioning features 51, 52, 53 are used to define the position of the key components (document 2, CCD 40, imaging lens 25,). These positioning features 51-53 may be molded into the housing 10, thereby fixing the relative positions of these components to a common, stable structure. Their description is summarized as follows.

Positioning 51 is used to hold the document 2 in its correct location relative to the imaging system 20. This feature is shown in FIGS. 15A, 16A and 16B.

The positioning feature 52 permits some axial motion of the imaging lens 25 to accomplish a combination of magnification adjustment and focus adjustment. The positioning feature 52 may also allow the imaging lens to tilt if center adjustment of the image on the CCD is to be accomplished through lens positioning. After the imaging lens 25 is properly positioned inside the housing, it is fixed in this position.

The positioning feature 53 includes mounts for the CCD 40 that permit adjustment of the CCD 40 in all three translational axes as well as rotational movement. This is done to achieve focus control, and to attain i) coincidence of the center of the image with the center of the CCD, ii) rotation about the optical axis to remove image skew from the image, and iii) rotation about the page scan axis of the CCD to compensate for image plane tilt error present in the imaging lens 25. The detail description of the positioning features 51-53 is as follows.

I. Positioning feature 51

The document 2, though being required to advance relative to the illumination system 70 and the imaging system 20, can be referenced to the imaging system 20 with the positioning features 51 of the housing 10 such that a portion of the document 2 is forced to lie perpendicular to the imaging lens' axis) OA, intercepting the object plane 3. (See FIGS. 16A, 16B, 16C). A preferred method of accomplishing this is to mold two rails 51A into the housing 10. These rails 51A are preferably placed outside the portion of the document 2 that is being imaged but such that the document rests on the rails 51A. The rails 51A are oriented so that the length of the rail coincides with the direction of document motion. The rail is preferably curved (convex) in the direction of document motion. If the document is forced into contact with these curved rails 51A (by tension or other means) such that it is slightly curled in the direction of its motion, the document 2 will be forced to be straight in the perpendicular direction. (FIG. 16B). That is, the document assumes a cylindrical shape in the region of the rails 51A. (FIG. 16C). Thus, the document 2 is forced to lie precisely in the curved plane defined by the curved rails 51A. The portion of the document being scanned therefore lies in a straight line 71 which is located in the front of the imaging lens 25, preferably about two focal lengths away.

The specific characteristics of the rails 51A are determined by the mechanical characteristics of the document, the wear and mechanical characteristics of the material of which the rails are made, and the force with which the document contacts the rails. The force required to contact the document to the rails is preferably generated by tensioning the document in the direction of document advance. A curved (cylindrical) surface 51B, 51B' may be used instead of the rails 51A to support the document 2. (See FIGS. 16D, 16E). Nevertheless curved rails are preferable to a similarly curved surface because such a surface will tend to scuff the image portion of the document and risk generating visual artifacts (such as scuff marks, scratches, or marks) that the rails arc able to avoid or minimize. However, either such surface or rails will force a document 2 to form a straight line in a direction perpendicular to the curvature.

It is preferred that the rails 51A be placed outside of the scanned region of the document to avoid producing visual artifacts in the critical area of the document. (See FIG. 16C). However, the rails should be placed as close together as possible to avoid buckling of the document into the void between the rails due to the tensioning of the document. The radius of curvature should be minimized within the constraints of the housing dimensions, since this will maximize the resistance to buckling of the document between the rails. For similar reasons, the rails 51A should not contain sudden changes in the radius of curvature (ideally they should be continuous curves), and they should be configured such that the ends of the rail are tangent to the tensioned document (FIGS. 16B, 16C). In the example illustrated in FIG. 16A, the length of the rails 51A is 10 mm, the width of the rails is about 1.5 mm, and the radius of curvature of the rails is about 25 mm. The rails 51A are separated from each other by approximately 18 mm (for APS format film, center to center distance). Distances would be different for different film formats. For example, for a 35 mm format, the distance would be larger. In the preferred embodiment, the rails are an integral part and molded with the rest of the housing. This reduces the number of parts required in the assembly and the variability associated with those parts. This also allows for the alignment of the imaging lens relative to the rails. Therefore critical alignment of the imaging system to a separate film track is not necessary. Fairly loose manufacturing tolerances, i.e. greater than 0.005", can be used for the assembly of the imaging system to the film track. Positioning feature 51 should be preferably constructed as concave or convex support for the document 2 (see FIGS. 16D, 16E), and may be configured as rails 51A or as a pressurized air shoe with air bleeder holes providing a moving air flow (FIG. 16E). Thus, the air shoe provides a moving air flow upon which the document floats, maintaining a consistent distance above the pressure shoe.

II. Positioning feature 52

The housing 10 also serves to stabilize the imaging lens 25 in such a way as to permit only axial motion (along the optical axis) of the imaging lens 25, thereby enabling compensation for manufacturing variations in focal length without lateral shift (i.e., a shift perpendicular to the line connecting the center of the document 2 and the center of the CCD 40) of the imaging lens 25. This is preferably accomplished by mounting the retroreflective lens 25 in a lens barrel 90 that is approximately 1.5× longer than its diameter D, and forming a bore 52A in the molded housing 10, such that the bore 52A is very slightly oversized (by approximately 1% of the lens barrel diameter) for this barrel. (See FIG. 17). It is preferred that at least three spring-loaded plungers (such as fingers 52B) contact the lens barrel 90 during alignment to ensure that the barrel makes stable line contact with the bore during axial adjustment. This ensures that the optical axis of the imaging system 25 is not altered during the focus adjustment step of the imaging system assembly. When final focus adjustment is achieved, the lens barrel may be firmly locked in place in the bore 52A (for example, by epoxy, mechanical fasteners, etc.), so that the fingers 52B may be no longer needed.

Alternatively, the plungers (such as spring fingers 52B) could be made of a material which, under load, maintain the contact pressure on the lens barrel 90 for the life of the imaging system (for example, insert molded steel fingers). In this case, no other locking mechanism is needed to secure the lens barrel in place. It is preferred to construct the spring-loaded plungers by molding "fingers" 52B into the wall of the housing 10. Such finger arc depicted in FIGS. 15C and 17. If these fingers 52B are molded with a pad 52C protruding into the bore 52A such that the pad 52C interferes with the outside diameter of the lens barrel 90), the fingers 52B will flex and exert a force upon the lens barrel such that the lens barrel is forced against the opposite surface of the bore 52A. The height of the pad 52C is determined by the mechanical characteristics of the molded housing material utilized, as well as the dimensions of the fingers and surface finish. In order to avoid catastrophic failure during assembly of the imaging system 20, care should be taken to design the size and shape of the spring finger 52B such that the stress induced by the bending of the finger is well below the yield point of the material. In other words, the height of the pad coupled with the length, width, and thickness of the spring finger determine the stress induced during assembly as well as the ability to hold the lens barrel in place. In the preferred embodiment, the material for housing 10, fingers 52B, and pads 52C is glass-filled polycarbonate. Furthermore, the higher the glass loading the more stable the imaging system becomes throughout the environmental changes. In the preferred embodiment, the material is a 30% glass filled polycarbonate. Moreover, if multiple fingers 52B with protruding pads 52C are utilized and arranged such that two protruding pads define a line parallel to the bore axis, and another finger exerts force at a point on the lens barrel 90 not on that line, the cylindrical lens barrel 90 is captured between two parallel lines and a single point. This condition is inherently stable, and the lens barrel 90 will not move perpendicular to the bore axis. The lens barrel 90 will slide axially maintaining contact with the two parallel lines, which are also parallel to the optical axis.

III. Positioning feature 53

The housing 10 may also provide a positioning feature, such as a reference surface 53A to locate the CCD. The number of constraints embodied in the positioning feature 53 to hold the CCD 40 in its precise location is determined by the degree to which the tolerances of the CCD 40 match the imaging system requirements. For instance, a particular CCD 40 may be manufactured to very tight mechanical tolerances permitting adequate optical alignment of the CCD 40 by referencing the CCD mechanical package to the mechanical references 53B of the positioning feature 53 of the housing 10. Alternatively CCDs may be manufactured with such loose mechanical tolerances that each of those CCDs must be optically aligned to the rest of the imaging system during assembly. This may be accomplished according to the procedure illustrated in FIG. 18A which is described below.

Alignment Procedure

The intent of the following procedure is to image a target 95 with the imaging system 20 onto a predetermined location on the CCD 40. An example of such a target is illustrated in FIG. 18B. This target enables a consistent criterion for alignment of the imaging system. In order to accomplish this task, it is necessary to design a target that will be used in place of the document (such as film or paper). The design of the target should take into account the magnification at which the imaging system will be working, so that very precise measurements can be made during the alignment process. The target must be of much higher optical quality than the imaging system, so that any degradation of the image will be immediately apparent. Furthermore, the target must be dimensionally stable, so that humidity and temperature have little or no effect or the dimensions of the pattern. The target should, as a minimum, possess an identifiable pattern or feature at the precise center of the illuminated document plane, and should provide sufficient numbers of sharp edged patterns (square wave patterns, for example) through out the field of view so that the best composite focus position may be determined, despite the presence of lens field curvature. This stability and precision can be attained, for instance, with a chrome target printed on a glass substrate.

The following are detailed steps of an alignment procedure:

1. Procure an appropriate (transmissive or reflective) target 95 that at least fills the illuminated object plane 3. The target 95 should have an identifiable pattern or feature that is positioned at the optical axis of the imaging system 25. This pattern is the center indicator.

2. The target 95 should be placed at the object plane with the pattern registered against the curved rails 51A or on a similar positioning feature.

3. Illuminate the target 95 with a suitable illumination source. This illumination source should have the same characteristics a,; the actual illumination source 70 used in the operational document scanner. The actual illumination source 70 that will be used in the assembled document scanner may be used for alignment purposes.

4. Procure a suitable CCD 40. The CCD 40 must be operational, and capable of delivering image data to a display or to a computer for analysis.

5. Position the CCD against the "z-axis" reference surface (positioning feature 53) using a manipulation mechanism. This manipulation mechanism should be also able to precisely: i) position the CCD in x and y directions, and ii) tilt the CCD by angles θ_(z) and θ_(y) (where θ_(z) is angle of rotation about the z axis and θ_(y) is the angle of rotation about the y axis).

The z axis corresponds to the optical axis of the imaging system, the x and y axes are defined as the axes perpendicular to the z axis and are mutually orthogonal. The x axis is perpendicular to the direction of document advance (scan direction), and the y axis is parallel to the direction of document advance (cross scan direction). The z axis reference surface is a physical surface on the housing 10 that fixes the location of the CCD 40 along the optical axis of the imaging system, but allows freedom of CCD translation in x, and y directions, and tilt in θ_(z).

6. Place the imaging lens 25 into the lens barrel 90. Insert the lens barrel 90 into the bore 52A of the housing 10 using a manipulation mechanism permitting precise positioning of the lens barrel in the z direction. Such a manipulation mechanism may be a moving pin inserted through a small aperture or slot 10D provided in a housing 10. (See FIGS. 15A, 15B).

7. Adjust (if necessary) the position of the CCD by translating the CCD in the x and y directions and using the signals derived from the active CCD 40. These signals will identify the position of the target image on the CCD 40. This may be done by a manipulation mechanism that is holding a circuit board 97 which is attached to a CCD 40.

8. Re-adjust the z position of the imaging lens 25 to optimize the sharpness of the image. Ensure that the imaging lens 25 is located such that the CCD 40 is located at the median position of the depth of focus. Lock the lens and barrel assembly 92 into position.

9. Re-adjust (if necessary) the CCD position in x direction until the image of the center indicator falls on the desired CCD element. Usually, the desired CCD element is the center element of the CCD array.

10. Re-adjust (if necessary) the CCD position in y direction until the image of the center indicator falls on the CCD array.

11. Re-adjust (if necessary) the CCD position in θ_(z), about the optical axis, until the CCD is properly aligned (angularly) to the target.

12. Adjust (if necessary) the CCD position by tilting it by the angle θ_(y) to minimize the effect of the field tilt (introduced by the imaging lens) along the length of the CCD.

13. Lock the CCD permanently into this position. A preferred method of fixing the CCD to the housing 10 after performing precision adjustment is using an ultraviolet (UV) curing adhesive. Viscous rivets 10F of this adhesive are formed by dispensing adhesive directly onto a surface of the CCD by an applicator, and continuing the dispensation as the applicator (such as a hypodermic needle 98) is slowly withdrawn through the aperture 10E in the housing 10. (See FIG. 19). The rate of flow of adhesive and the rate of applicator withdrawal may be coordinated such that a rivet-shaped column of viscous adhesive contacts both parts. Ultraviolet exposure of the adhesive rivet 10F for several seconds will harden the adhesive and firmly lock the relative position of the CCD and housing 10. This is described in detail in the above referenced patent application Ser. No.(Attorney Docket 77289FHB), entitled "ULTRAVIOLET CURABLE RIVETING OF PRECISION ALIGNED COMPONENTS".

The CCD captures an image of a document and generates an analog representation of the image information. After being converted into digital format, the image information is sent to an image receiver 100. Depending on the application this image information may or may not be modified and recorded to a variety of receivers. Such receivers may include paper, film, photosensitive drum, magnetic tape or digital file.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

    ______________________________________                                         PARTS LIST                                                                     ______________________________________                                         1            Document scanner                                                  2            Document                                                          3            Object Plane                                                      5            Image plane                                                       10           Housing                                                           10A          First housing part                                                10B          Second housing part                                               10C          Slit in the housing                                               10D          Aperture                                                          10E          Aperture                                                          10F          Rivet                                                             20           Imaging System                                                    25           Imaging Lens                                                      25A          First Lens Element                                                25B          Second Lens Element                                               25C          Reflective surface                                                ST           Aperture Stop                                                     30           Folding Mirror                                                    40           CCD                                                               51           Positioning fixture                                               51A          Rails                                                             51B          Curved cylindrical surface (convex)                               51B'         Curved cylindrical surface (concave)                              52           Positioning fixture                                               52A          Bore                                                              52B          Fingers                                                           52C          Pads                                                              53           Positioning fixture                                               53A          Reference surface                                                 55           Baffle                                                            60           Analog to Digital converter                                       70           Illumination System                                               71           Illumination line                                                 75           Flange                                                            80           Base                                                              90           Lens Barrel                                                       95           Target                                                            97           Circuit board                                                     98           Applicator                                                        100          Receiver                                                          ______________________________________                                     

What is claimed is:
 1. A retroreflective lens, said retroreflective lens comprising:only two lens elements wherein said two lens elements include a single positive power lens elements with and index of refraction N₁ and a thickness T₁ ; and a single negative power lens element with and index of refraction N₂ and a thichness T₂, the negative power lens element being spaced apart from the positive power lens element by a distance S₁ ; and a reflective surface intercepting light propagating from the negative lens element and reflecting this light toward the negative lens element; wherein N2>1.6, N1-N2>0.15, and said lens elements in combination with said reflective surface provide said retroreflective lens with an infinity F-number between F/3 and F/6.
 2. A retroreflective lens according to claim 1 wherein T1+T2+S1<0.15 F, where F is the focal length of the retroreflective lens.
 3. A retroreflective lens according to claim 1 wherein said retroreflective lens has an aperture stop located in a vicinity of said reflective surface.
 4. A retroreflective lens according to, claim 1 wherein the positive power lens element is biconvex and the negative power lens element is biconcave.
 5. A retroreflective lens according to claim 1 wherein said retroreflective lens has a focal length and wherein a fractional focal length change Δf will result in a fractional change of magnification ΔM such that |ΔM|<|Δf|.
 6. A retroreflective lens according to claim 5 wherein |ΔM|<0.2|Δf|.
 7. A retroreflective lens according to claim 1 wherein said retroreflective lens is adapted to image at a magnification M, wherein 0.9≦|M|≦1.1.
 8. A retroreflective lens, said retroreflective lens comprising:only two lens elements wherein said two lens elements include a single positive power lens element with an index of refraction N₁ and a thickness T₁ ; and a single negative power lens element with an integral reflecting surface, an index of refraction N₂, and thickness T₂, the negative power lens element being spaced apart from the positive power lens element by a distance S₁ ; wherein N2>1.6, N2-N1>0.12, and said lens elements in combination with said reflective surface provide said retroreflective lens with an infinity F-number between F/3 and F/6.
 9. A retroreflective lens according to claim 8 wherein T1+T2+S1<0.2 F, where F is the focal length of the retroreflective lens.
 10. A retroreflective lens according to claim 8 wherein said retroreflective lens has an aperture stop located in a vicinity of said reflective surface.
 11. A retroreflective lens according to claim 8 wherein the positive power lens element is biconvex and the negative power lens element is a meniscus lens element with a convex reflective surface.
 12. A retroreflective lens according to claim 8 wherein said retroreflective lens has a focal length and wherein a fractional focal length change Δf will result in a fractional change of magnification ΔM such that |ΔM|<|Δf|.
 13. A retroreflective lens according to claim 12 wherein |ΔM|<0.2|Δf|.
 14. A retroreflective lens according to claim 8 wherein said retroreflective lens is adapted to image at a magnification M, wherein 0.9≦|M|≦|1.1. 